Receiving device



Oct. 11, 1938.

F. BLYTHEN ET AL 2,133,075 RECEIVING DEVICE Filed Feb. 12, 1934INVENTORS' I FRANK BLYTHEN m 101% HA x20 W/CK I I M ATTORNEY.

Patented Oct. 11, 1938 UNITE STATES PATENT orrics RECEIVING DEVICEApplication February 12, 1934, Serial No; 710,802 In Great BritainFebruary 13, 1933 3 Claims.

The present invention relates to cathode ray tubes such for example asare used in television receivers.

Such a tube may comprise a heated cathode, a

5 control electrode, means for accelerating the elec tron ray andbringing the electrons to a focus on a fluorescent screen and means fordeflecting the ray over the screen.

The received picture signals are applied between the control electrodeand cathode, to vary the negative potential of the grid in relation tothe cathode, and serve to increase and decrease the intensity thereofwhilst the ray is swept, by the deflecting means, over the screen inharmony with the scanning device at the transmitter.

In such tubes it is found that the light intensity-grid voltagecharacteristic is very curved, the rate of change of light intensitybeing considerably greater for less negative grid potentials,

that is higher light intensities, than for more negative gridpotentials, that is lower light intensities. It is usually necessary tobias the control grid relatively to the cathode in such a way that apicture signal of maximum amplitude in the 5 black direction reduces theintensity of the ray substantially to zero and when this is done it isfound that, owing to the curvature of the characteristic, gradations ofintensity in the darker parts of the reproduced picture are masked 30whilst, in the lighter parts, they are unduly accentuated.

It is an object of the present invention to remove or reduce the errorsintroduced by a nonlinear characteristic in a cathode ray tube.

5 It is a further object to provide a method of transmitting andreceiving images of objects wherein picture signals are distorted,either at the transmitter or at the receiver, before being applied to anon-linear reproducing device such as 40 a cathode ray tube, in suchmanner as to compensate, at least partially, for the non-linear lightintensity-modulating voltage characteristic of said reproducing device.

Yet another object of the present invention is 45 to provide televisionreceiving apparatus comprising a cathode ray tube or a like reproducingdevice having a non-linear light intensity-modulating voltagecharacteristic and means for distorting picture signals, beforeapplication to said 50 device, in such manner as to compensate, at leastpartially, for the non-linearity of said device.

Yet another object of the present invention is to provide televisiontransmitting apparatus comprising means for generating picture signalscor- 55 responding to the light and shade of the object and means forincreasing the amplitude of signals representative of the darkerportions of the object as compared with signals representative of thelighter portions of the object.

An embodiment of the invention will now be described with reference tothe accompanying drawing in which Fig. l is a circuit diagram oftelevision receiving' apparatus arranged in accordance with the presentinvention, 16

Fig. 2 is a graph of the light intensity-grid, voltage characteristic ofa known form of cathode ray tube, light intensity being plotted asordinates and grid voltage as abscissae, and

Fig. 3 is a graph of the anode current-grid volt- E15 age characteristicof a thermionic valve, anode voltage being plotted as ordinates andgridvoltage as abscissae.

Referring now to Fig. 1, the received picture signals are applied to thegrid circuit of a screen- 20 grid valve I, the anode circuit of thisvalve being coupled to the control grid circuit of a cathode ray tube 2.The positive potential applied to the screening grid 3 relative to thecathode 4 of the valve l and the value of the anode resistance 5 areboth lower than usual. By suitably biasing the control grid 6 of thevalve l relative to the cathode 4, the shape of the anode currentgridvoltage characteristic of this valve, shown in Fig. 1, can be madecomplementary to the shape of the light intensity-grid voltagecharacteristic of the cathode ray tube 2, shown in Fig. 3, so that thesignals applied to the grid 1 of the cathode ray tube 2 are distorted,signals in the black direction appearing in the anode circuit of thetube 2 at greater intensity than signals in the light direction.

The valve I is used also to rectify an incoming carrier modulated withpicture signals. This is done by arranging the valve, in the mannerillustrated, as a grid-leak rectifier.

When the distortion introduced by the cathode ray tube 2 is compensatedfor in the manner described, the negative bias upon the control grid 1of the cathode ray tube 2 may be made less than is usually necessarywhilst still obtaining good reproduction of black portions of a picture.As a consequence of this, an increase in modulation does not producesuch an increase of background brightness as results from working withhigher values of bias. Any change that remains may, however, becompensated for by biasing the grid 1 of the cathode ray tube 2, asshown, from the D. C. potential drop along the anode resistance of thedistorting valve.

For the reception of a carrier frequency of about 40 megacycles thefollowing arrangement is satisfactory:

A selector circuit 8 tuned to the carrier frequency to be received hasone terminal connected to the cathode 4- 0f the screen-grid valve l andthe other terminal connected through a condenser 9 of 25micromicrofarads shunted by a resistance ID of 10,000 ohms. Thescreening grid 3 is maintained at a potential of 40 volts relative tothe cathode 4 and the anode H is connected through a resistance 5 of10,000 ohms to a point at 200 volts relative to the cathode 4. The endof the resistance 5 remote from the anode H is connected to the cathodel2 of the cathode ray tube 2 and the other terminal of the resistance 5is connected through a high frequency choke coil I4 to the control gridI of the cathode ray tube 2.

A receiver of this kind is of particular advantage for the-reception ofmotion picture films because it is common practice in the film industryto develop films in such manner that detai in bright portions of thepicture is brought out, or amplified, as it were, relatively to thedetail in the darker or more opaque portions of the pictures. This isdone because pictures projected upon a screen from films developed inthis way appear more natural to the eye. Therefore it is essential that,when films of this kind are transmitted, the receiver should not stillfurther emphasize detail in bright portions of the picture, as would bethe case if distortion of the received signals is not effected as isdescribed herein.

Although it is preferred to carry out the correction of the picturesignals according to the present invention at the receiver, in somecases the correction may be effected before transmission.

We claim;

1. The method of compensating for distortion in reproducingelectro-optical images upon the viewing screen of a cathode ray tubehaving normally non-linear light intensity modulating voltagecharacteristics which comprises the steps of applying from an amplifyingsystem to the control circuit of the electro-optical reproducing devicecontrolling signals and modifying the signals prior to applying the samefor control purposes by operating the amplifying system upon thatportion of its anode current-grid voltage characteristic which has acurvature substantially complementary to the curvature of the lightintensity modulating voltage characteristic of the electro-opticalreproducing device.

2. The method of compensating for distortion in reproducingelectro-optical images upon the viewing target of a cathode ray tubehaving normally non-linear light intensity modulating voltagecharacteristics which comprises the steps of amplifying and non-linearlydetecting controlling signals to provide an output characteristiccomplementary to the non-linearity of the cathode ray tube and applyingthe amplified detector signals to the control circuit of the cathode raytube to modulate the electron beam developed in the tube whereby asubstantially linear response is obtained.

3. In a television system wherein a non-linearly responsive cathode raytube having a control circuit is used to produce electro-optical imagerepresentations andwherein a grid leak multi-grid thermionic tube havinginput and output circuits is arranged to receive upon the input circuitsignals representative of the electro-optical effects, the method stepsof operation which comprise non-linearly detecting the received signalsin a manner complementary to the nonlinearity of the cathode ray tubeand feeding the detected signals from the output circuit of thethermionic tube through a frequency discriminating path to the controlcircuit of the cathode ray tube so that electro-optical imagerepresentations are substantially linearly reproduced.

. FRANK BLYTHEN. JOHN HARDWICK.

